The definition of ruthless is someone without compassion or who doesn't show mercy.
An example of ruthless is someone running over a dog with their car on purpose.
having or showing no pity; merciless; relentless: a ruthless competitor
Origin of ruthlessMiddle English reutheles
Having no compassion or pity; merciless: ruthless cruelty; ruthless opportunism.
(comparative more ruthless, superlative most ruthless)
ruth +"Ž -less